25.4-31.8 bar shim safety factor

Anybody have any experience using a shim for 25.4 bars for an OS stem? I am in the process of building up my old steed, a giant vt, for a buddy who is returning to the states and is interested in mtn biking - he has never had a dual suspension bike, so I thought it would be a nice surprise upon his arrival to have a dually ride for him. Plus, I don't want our first ride together to be on his 7 year old hardtail while I am on my 5 spot - kinda unfair and not that fun for him. He is a couple inches shorter than I am, so I am attempting to shorten the cockpit a bit by using a 50mm stem on the vt, but don't have the correct sizing bar wise. Do I man up and buy a new stem or bar, not knowing his preference yet, or is this shim an ok idea? If it matters, it is a Giant made shim from the LBS. Last thing I want is to welcome a buddy home from some serious sh*t overseas only to give him a faulty bike. My main goal here is to allow him to get used to the bike and for me to have some fun rides with my buddy without a)injury and b)major outlay $$ wise until he knows what he wants from the bike.

Thanks for the advice - I had asked the LBS about it when buying the shim, but their response was not very comforting - along the lines of "what? oh yeah, it'll work fine, dude!" Now that I have assembled the bar, shim and stem it feels fine, but I figured somebody would have some knowledge about it above and beyond the LBS.